I've got 18" 205 width tyres on my car but it touches the rear fender lip ever so often or when I've got passengers in the back seat.I'm thinking of rolling the rear lip but I'm told this can damage the paint and also water and dirt can collect in there when you roll it and cause the fender to begin rotting.Is cutting off say half the lip a safer option? What does it do to the strength of the fender?

If you roll them properly as in with an arch roller and heat the arches as your doing them the paint should be fine and not flake and crack.Cutting the lip won't do anything to the strength of the fender but I guess it depends on how much of the lip is rubbing if its only a small amount I would of thought just slightly rolling them would be more than enough to stop it, but if it's more then you may have to roll them more to get the clearance you need.Good luck with it

To be honest I don't think I've ever heard of that happening to anyone. I guess there's a chance of it happening but it's not going to happen over night, it would take a fair few years to rot it out.I'd say if you really are that worried about your wings rotting out then you could always just either paint the inside of the lip or use underseal on it to stop any kind of rust appearing